1. Field of the Invention
This invention relates to an image reader for image processing and, more particularly, to an image reader for reading characters or images and generating a corresponding digital image signal (e.g. a digital scanner or a digital copier). Such image readers comprise a light source for exposing an image to light, and an image sensor (e.g. a CCD image sensor) for generating a corresponding image signal, and means for digitizing the image signal and digitally processing the digitized image signal.
2. Description of the Prior Art
A digital image reader is disclosed in our European Patent 0267793B granted to the present Asignee. In a reader of this type, an image of characters or pictures written or drawn on pages of a book, or some other document, or alternatively recorded on a microfilm or other transparent medium, is read using a CCD image sensor to provide an image signal which is then either recorded, processed or transmitted.
One example of an image reader is arranged to read an image from microfilm, and to print a corresponding output copy, and comprises a CCD image sensor or the like, means for electrically processing the image sensor output, and means for providing a digital signal to a printer unit such as a laser beam printer to obtain a copy. FIG. 14 is a block diagram of an image reader provided within such a reader/printer apparatus. In FIG. 14, reflected light from an original placed on, for example, an original table or light projected through a microfilm, is scanned by an image pick-up section 21 comprising an image pick-up device such as a CCD, and is converted into an electrical signal which is quantized into a multi-valued signal by an A/D conversion section 22. Multi-level quantized data thereby obtained is corrected by a gamma correction section 29 to take account of the difference between the photo-electric conversion characteristic of the image pick-up device in the image pick-up section 21 and the human gradation perception characteristic, illustrated in FIGS. 15, 16 and 17.
FIG. 15 shows an example of the photo-electric conversion characteristic of an image pick-up device (e.g. a CCD device). The abcissa represents a density scale which is linear with respect to the human gradation perception characteristic. It will be seen that the photo-electric conversion characteristics of image pick-up devices in general differ from the human gradation perception characteristic. For this reason, a gamma correction table embodying a correction characteristic such as that shown in FIG. 16 is used to provide density correction or density conversion, so that the output D' is linear with respect to the human gradation characteristic as shown in FIG. 17. The gamma correction characteristic is therefore essentially the reverse of the conversion characteristic of the image pick-up device with respect to the human gradation perception characteristic.
The signal processed by the gamma correction section 29 is converted, by a gradation processing section 25, into binary (bi-level) image data from which a graded image can be reproduced by, for example, a dither or error diffusion method, and the processed data is output from the reader for subsequent printing or other processing or transmission.
Microfilm images are provided either as negative images or positive images on corresponding negative or positive films, which are selectively used for different purposes. A known image reader enables either negative or positive films to be copied; in this reader apparatus an image from the film is directly projected onto a photo-sensitive member from which it maybe printed (e.g. electrostatically). This direct, or analog, type printer therefore has two separate image forming processes, one for negative film and one for positive film, and requires toners having different polarities and two different development devices, a changeover mechanism for interchanging between the two, a circuit or mechanism for changing the polarity of a high voltage output from a transfer charger or other charger, and two separate high voltage power sources for different load characteristics, and requires also a changeover for providing blanks for blank exposure. Analog type reader/printers of this type are proposed by the present asignee in U.S. Pat. No. 4,341,463 and U.S. Pat. No. 4,627,703. This type of apparatus can be used to provide images having good gradation characteristics (grey scale reproduction) by selecting the reproduction voltages for each of the positive and negative film processes so as to provide optimal gamma characteristics (exposure/density characteristics). However, since two separate developing processes and associated changeover mechanisms are required, the apparatus is necessarily undesirably complex.
Digital reader/printers (for example the Canon DMP100) are, on the other hand, less mechanically and electrically complex. In principle, such a reader/printer can be used to obtain a positive print copy from a film of either type merely by selectively inverting the digital signal output. However, if the digital signal is merely inverted in this way, it is not possible to obtain an image having good gradation characteristics. One of the reasons for this is that the gamma characteristic of a film is non-linear and, referring to FIG. 9, curved portions of the characteristic in the vicinity of A and B are not symmetrical.
Analog type reader/printers of this type are proposed by the present asignee in U.S. Pat. No. 4,341,463 and 4,627,703.
Thus, when a positive original is photographed onto a film, the portion A corresponds to a line drawing portion and the portion B corresponds to a background image portion whereas when a negative original is photographed onto the film, the portion B corresponds to the line drawing portion and the portion A corresponds to the background portion. Thus, if a reader/printer with a gamma correction characteristic set for one type of original is used to read and print the opposite type of original, by merely inverting the digital image data, the result is that line image portions receive the gamma correction appropriate for background portions and vice versa.
Japanese laid open applications nos. 3-150977 and 3-160877, and corresponding U.S. applications Ser. No. 604,955 filed on 25th Oct. 1990, all assigned to the present assignee and published after the present priority date, describe a reader/printer in which this problem is solved by providing means for selecting the gamma correction in dependence upon whether the film is a negative or positive original film.
In such a digital reader/printer, ordinarily, the signal from the image pick-up device is amplified and digitized, and a look-up data table is used for converting the digitized data to gamma corrected data, the look-up table operating in accordance with a linear or non-linear scale, which in either case does not have a one-to-one output characteristic.
In general, in the case of processing image data from a microfilm (or like transparent medium) the print density level required is found to depend not only on the non-linearity of the gamma characteristic of the film but also on the original type (positive or negative), the density characteristic of the original image (imaged by a camera), the resolution of the camera lens which imaged the original, the extent of defocus thereof, the kind of film (silver salt, diazo, vescicular and so on) the film developer liquid, the development conditions, and other factors, as well as the purpose and the user's preference.